1. Field of the Invention
The present invention relates to a method of removing methane and other gases from subterranean coal deposits or other mineral deposits by horizontal drilling of degasification holes in said deposit.
2. Brief Description of the Prior Art
The inclusion of large quantities of methane in coal deposits long has been a safety problem in many areas of the world. The methane is tightly absorbed in the coal micropores and on the coal surfaces and is released during mining which creates a safety hazard.
There have been many attempts to overcome the problem of methane in coal deposits in the past. Early attempts to overcome the problem involved drilling a series of vertical vent holes in the deposit in the hope that the methane would flow from the coal deposit out of the vent holes. Controlled slant hole drilling through the overburden and into the coal deposit also has been attempted. More recent attempts have included such things as applying vacuum to the coal deposit to accelerate methane removal and the introduction of a displacing fluid such as a gas or water into the coal deposit to displace the methane.
U.S. Pat. No. 4,043,395 describes a method for removing methane from a coal deposit by injection of a carbon dioxide-containing fluid through an injection well extending into the coal deposit. The well then is shut in for a time sufficient to enable a substantial amount of absorbed methane to be desorbed into the injection fluid. The injection fluid with desorbed methane then is recovered from the injection well or separate wells spaced from the injection well. This process is repeated until the methane level in the coal is reduced to a level suitable for safe coal mining.
Most recently, with deeper mines being the current trend, that is mines 1250 feet to 2500 feet below the surface, horizontal boreholes drilled into a virgin coal deposit from a vertical entry shaft have become a viable method of draining methane from the coal deposit prior to mining development. Equipment and methods for drilling long holes in coal with reasonable directional control have been virtually nonexistent. Technology now permits the successful drilling of initial horizontal holes 500 to 1000 feet in length from the bottom of a coal mine ventilation shaft which is projected in advance of mine entry development. To date, this drilling has been performed with either specially constructed or modified rotary drills. While many survey instruments are available for determining the position of a borehole, they each require drilling to be discontinued and the survey tool to be pumped down the drill string to the position to be surveyed. The survey tool is removed by withdrawal of the drill string or by use of a wire line attached to the end of the survey tool. Survey instruments that are attached to the end of the drill string and that transmit data by cable previously have been considered unfeasible by those skilled in the art.
While all of the previous attempts have been successful in some degree, they have not been completely satisfactory due to the inadequate removal of methane or due to the excessive time required to carry out these processes. The rate of advance in working coal seams has been greatly increased with the advent of mechanization in underground mining and particularly long wall mining of coal deposits. The more rapidly advancing working face in the mining operation results in a constant release of methane from the coal due to the release of rock pressure and crack formations connected therewith. For this reason, in order to maintain adequate safety standards, operations periodically must be interrupted while steps are taken to maintain the concentration of methane gas below the permissible maximum. The interruption of the mining operation is undesirable for both technical and economic reasons.
To provide satisfactory drainage of the methane while permitting development to continue, substantially longer boreholes are required to degasify larger areas of the virgin coal. The present survey and drilling methods lack suitable accuracy to effect such degasification. It would be desirable to provide a process whereby methane could be removed from an area of an underground coal deposit prior to development of the mining entries thereinto while permitting mining to continue in other areas.